The sedimentologic features that characterize the pinchout of Mesaverde Group sandstones into the Mancos Shale are spectacularly exposed along the Book Cliffs in east-central Utah. We traced the upper portion of the Blackhawk Formation of the Mesaverde Group as it graded into Mancos Shale, and observed textural trends and sedimentary structures which match those that are observed in modern tempestites formed on siliciclastic shelves. The upper-most portion of the Mancos consists of upward coarsening sequences of mudrocks that exhibit large-scale scour and fill structures and slump features. Above the Mancos, sandstone units of the Blackhawk Formation commonly display sole marks within partial Bouma sequences, and sandstone units higher in the section consist of hummocky c oss-strata sets grading up to asymmetrical ripple sets. These units represent tempestites that were deposited by wave-modulated, wind-driven currents generated by storms as they crossed the mud-dominated western shelf of the Cretaceous seaway. Tempestite sequences can be traced up depositional dip into tongues of prograding shoreface sandstones.

The moving of large quantities of sand and silt-sized siliciclastics out onto mud-dominated shelf environments appears to be a major process along margins of epicontinental seaways. We believe that these movements are storm-generated, and that sands, which later became petroleum reservoirs, were placed in shelf settings in the form of tempestites. In particular, many petroleum-bearing Cretaceous sandstones in Wyoming, Colorado, and Utah that have previously been interpreted as either deltaic-coastal bars or "shallow-water turbidites," are actually tempestites.

End_of_Article - Last_Page 489------------